Upgrade rust/crates/structopt to 0.3.20 am: be338a19d4 am: 16ba8063d3 am: 71dc28ca2c

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/structopt/+/1474997

Change-Id: I312db6cf3ba20b0894bd1ab8f607b3bf973dc18d